Introduction and General Aspects

An appreciation has been written of the seminal, major contributions of H.S. Isbell to carbohydrate chemistry.

Several carbohydrate applications have been included in a review of microwave-assisted organic reactions, and a survey of chemical reagents in photo-affinity labelling included the role of various base-labelled azido- and thio-nucleosides, sugar azide derivatives and p-benzoylbenzoate esters of nucleosides. A further review intitled 'Reverse Anomeric Effect: Fact or Fiction?' refers to early evidence based on studies of glycosyl pyridinium and imidazolium salts. It concludes that its origin as an electronic effect is not supported by theory or experimental results.

A review in 'Advances in Carbohydrate Chemistry and Biochemistry' has covered 13C-1H coupling in sugar derivatives and included theoretical aspects, experimental techniques and conformational dependencies. A related survey was produced on 13C nuclear magnetic relaxation and motional behaviour of carbohydrates in solution. Theoretical and observed data were included.

Part I of a review on the uses of enzymes in carbohydrate chemistry has appeared, this part dealing with biological recognition and syntheses of monosaccharides and analogues including amino-sugars. A further essay on applications of enzymes deals with the use of glucansucrase in the synthesis of oligosaccharides and polysaccharides. The chemotherapy of HIV infection has been surveyed with attention being given to the various applications used and targets identified. Nucleoside analogues and glucosidase inhibitors were discussed amongst the drugs used.

An in-depth treatment of the organic chemistry of the monosaccharides also briefly covers their role in natural products, for example glycosides, oligosaccharides, polysaccharides and glycoproteins. Such topics as the chemical synthesis of oligosaccharides and the use of sugars in the synthesis of enantiomerically pure non-sugar products are dealt with after an extensive mechanistic treatment of the reactions of the monosaccharides. Containing 1000 references, the book provides a convenient means of access to the primary literature.

Free Sugars

1 Theoretical Aspects

A sytematic search has been made for all low energy structures of α- and β-D-glucopyranose, α- and β-D-galactopyranose, β-D-talopyranose and β-D-allopyranose in the solid state, the energies being minimized with respect to nine lattice and rigid-body parameters and six intramolecular dihedral angles. In four cases, the observed crystal structures corresponded to one of the lowest energy structures; in the other two cases, the observed structures were of more than 20 kJ mol-1 higher energy than the calculated minima. An extension of the GROMOS force field allowed an improved crystal structure determination of α-D-galactopyranose.

The hydroxyl acidities of sucrose, assessed through semiempirical calculations of the deprotonation enthalpies, followed the order OH-2g [much greater than] OH-3g > OH-3f > OH-1f = OH-4g > OH-4f [much greater than] OH-6g > OH-6f. The molecular electrostatic potential profile of sucrose in polar, aprotic solvents indicated likewise that in the main conformation OH-2g is the most electropositive hydroxyl group; the preparations of selectively benzylated and acetylated sucrose derivatives on the basis of these findings are referred to in Chapters 5 and 7, respectively.

The hydration of α-maltose has been investigated using molecular modelling and thermodynamic methods. Due to crystallinity, the observed non-freezing water content was lower than that calculated.

Conformational studies on several free sugars are covered in Chapter 21.

2 Synthesis

A review article on enzymatic oxidoreduction in organic synthesis included the following carbohydrate-related topics: i) oxidation of polyols to polyhydroxy aldehydes by use of galactose oxidase (yields 10-15%); ii) the reduction of 2-acetylfuran by alcohol dehydrogenases from two different sources to furnish enantiospecifically either the (R)-alcohol 1 or its (S)-isomer 2, which serve as potential building blocks for the synthesis of D- and L-sugars, respectively.

Aldolase- and transketolase-catalysed synthesis of pentoses, hexoses and higher sugars continues to receive considerable attention, especially by C.-H. Wong and co-workers who have covered these processes in Part 1 of a review article (60 refs.) on the use of enzymes in carbohydrate chemistry. DERA (deoxyribose 5-phosphate aldolase, EC 4.1.2.4) has been prepared on a large scale by recombinant techniques. It was employed, either by itself or in combination with RAMA (rabbit muscle aldolase, a fructose-1,6-diphosphate aldolase, EC 4.1.2.13) or NeuAc aldolase (EC 4.1.3.3 ) and, if necessary, phosphatase in sequential condensations to afford deoxyaldoses from -pentoses to -nonoses from small substrates such as acetaldehyde, glyceraldehyde and dihydroxyacetone phosphate. Condensations catalysed by L-fuculose 1-phosphate aldolase (EC 4.1.2.17) and L-rhamnulose 1-phosphate aldolase (EC 4.1.2.19), in combination with phosphatase and the appropriate isomerases, resulted in efficient syntheses of L-fucose and L-glucose, respectively. An alternative strategy to prepare aldoses by this process which initially produces doses, is illustrated in Scheme 1. Similar condensations involving the masked hydroxydialdehydes 3 and 4 were used in the synthesis of aldoketose derivatives 5 and 6, respectively, required as precursors for the microbial preparation of deoxynojirimycin analogues. The condensation of pyruvate with simple hydroxyaldehydes, e.g., D-glyceraldehyde or D-erythrose, under catalysis by an aldolase from Aspergillus terra gave 3-deoxy-2-ulosonic acids with (R)-configuration at the new chiral centre (C-4).

2.1 Tetroses and Pentoses. – The application of the Strecker synthesis to the preparation of 2-amino-2-deoxytetrose derivatives is covered in Chapter 9 and a lipase-mediated route to 4-carbon diols and triols is referred to in Chapter 18. The CaCl2/KOH-promoted aldol condensation of dihydroxyacetone with formaldehyde has been optimized for the...